1. Field of the Invention
The present invention relates to a filter apparatus, and specifically, for example, relates to a filter apparatus for use in a cellular telephone of and other radio equipment.
2. Description of the Prior Art
In recent years, in the United States, following a rapid propagation and expansion of cellular car telephones and a rapid increase in the number of cellular base stations, extension of the width of frequency bands to be used has been studied to increase the number of channels to be used, and assignments of frequency bands to be used have been announced. In this situation, the frequency to be used in a service system sometimes approaches the frequency to be used in another service system, and therefore, to prevent mutual interference between those service systems, for example, a filter apparatus having a sharp characteristic as shown in FIG. 14 and a filter apparatus having a sharp characteristic as shown in FIG. 15 are requested.
To obtain such a sharp characteristic, it has been considered to configure a filter apparatus using a cavity resonator, but this apparatus becomes large in shape, going against the request for miniaturization.
Also, conventionally, in a filter apparatus using resonators, to obtain such a sharp characteristic, the value of Q is required to be increased by coupling each active circuit to each resonator respectively, resulting in a complicated structure.
Furthermore, in the conventional filter using resonators, a load is connected to the output end thereof, and therefore the value of Q.sub.0 of the resonator has to be made considerably large taking the effect of the load into account. In such a case, an active circuit coupled to the resonator has to be operated in an unstable state, and thereby the apparatus lacks practical usability.